1. Field of Invention
This invention relates to an electronic tachometer and associated automatic speed control apparatus which is particularly useful in magnetic disk drive data storage devices of the kind now commonly used in electronic data processing systems and is described herein with reference to such a device.
2. Description of Prior Art
As is well known to those skilled in the art, a disk drive typically comprises a disk pack consisting of a plurality of magnetic recording disks each having a multiplicity of concentric recording tracks and being mounted on a drive spindle in stacked slightly spaced relation to one another for common rotation about the spindle. The disk drive further comprises an array of magnetic heads disposed in read/write relation with the disks. In the case of moveable head drives, the heads are mounted on an actuator driven carriage mechanism with at least one head operatively associated with each magnetic surface.
In some earlier disk drives, head position was determined by means of apparatus which included an optical grating mounted for movement with the carriage mechanism and operating in combination with a light source and associated light detector fixedly positioned to each side of the grating. As the grating moved with the carriage mechanism the light passing through the grating was modulated and the distance traveled by the head determined by detecting the modulated light, thereby providing an accurate indication of head position relative to the tracks on the disks. The head velocity was then determined by differentiating the head change of position signal. An example of such a head positioning system is disclosed in U.S. Pat. No. 3,597,750, Servo with AGC for Positioning a Magnetic Head, issued Aug. 3, 1971. Unfortunately, the velocity signal obtained with this system, though accurate was not continuous.
A continuous indication of head velocity has been provided by apparatus of the type disclosed in U.S. Pat. No. 3,568,059, Electronic Tachometer, issued Mar. 2, 1971. That tachometer provides a continuous indication of head velocity by integrating the current applied to the actuator which drives the carriage mechanism. It was recognized, however, that the actuator drive current was not totally accurate for indicating head velocity because of variations in wind resistance and friction associated with the carriage mechanism and other such factors. Therefore, the velocity signal obtained by integration of the actuator current had to be corrected periodically in order to obtain an accurate indication of head velocity. Such correction was provided by periodically adjusting the continuous actuator derived velocity signal to the value of a discontinuous but accurate velocity signal obtained by differentiation of a distance signal derived from an optical grating.
Other electronic tachometers incorporating optical grating apparatus for providing position and velocity information are known in the art and indeed are capable of providing a continuous velocity indication. A tachometer of this type is disclosed in U.S. Pat. No. 3,811,091, issued May 14, 1974. In any event, in presently used disk drives the optical grating is eliminated by the provision of position control servo data recorded directly on the magnetic disks. This reduces the cost of the disk drive and facilitates interchanging of the disk packs without concern for extremely precise positioning of the pack as is required with a grating apparatus because the position of the heads is referenced directly to the disks. Position control by use of such servo data may be achieved, for example, by a servo head which is operatively associated with one of the magnetic surfaces and affixed to the carriage mechanism to move in unison with data heads associated with the other magnetic surfaces of the disk pack. As the actuator mechanism moves the array of heads across the disk surfaces, the servo head responds to the servo data to produce a change of position or distance signal which is used for controlling the motion of the heads to locate a selected head adjacent a desired track of its associated disk. This distance signal, however, is not as accurate as the distance signal provided by a grating system and as a result a velocity signal derived therefrom is not accurate to the degree desired for high track density disk drives. More specifically, the distance signal obtained by detecting recorded servo data is usually truncated in shape or otherwise distorted so as not to have a constant amplitude, much less a predictable waveform. Since the velocity signal is generated by differentiating the distance signal, any change in the shape of such signal results in a change in its slope and thus provides an erroneous velocity signal. Changes in the slope of the distance signal can be caused, among other things, by variations in the amplitude of the recorded servo signal, variations in the coercivity of the magnetic recording surface and variations in the head flying height relative to the magnetic surface. The accuracy of the velocity signal is also affected by defects in the recording media and by the wider band width and higher slew rate required of the distance detecting and differentiating circuits used in high track density devices. Media defects, for instance, produce noise in the detected distance signal and in the case of high track densities may result in complete obliteration of certain tracks with a resultant substantial increase in noise and degradation of signal to noise ratio. Moreover, media defects adversely affect a velocity signal obtained by differentiation of a distance signal derived from recorded servo data because such defects are characterized by high frequency components which are accentuated by the differentiation process. The slew rate and band width of the differentiator circuit have a deleterious affect on the signal to noise ratio in high track density devices because the frequency of the distance signal obtained by detection of the servo data is proportional to the rate at which the head traverses the tracks which in turn is dependent on the track density.
The foregoing and other disadvantages and limitations of electronic tachometers presently used in high track density disk drives are overcome by the present invention as will become apparent from the description provided hereinafter, particularly of the preferred embodiment given with reference to the appended drawings.